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Bureau of Mines Information Circular/1984 



Review of Anhydrous Zirconium-Hafnium 
Separation Techniques 

By Robert L. Skaggs, Daniel T. Rogers, and Don B. Hunter 




2 UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8963 



Review of Anhydrous Zirconium-Hafnium 
Separation Techniques 

By Robert L. Skaggs, Daniel T. Rogers, and Don B. Hunter 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 



1 




Library of Congress Cataloging in Publication Data: 



Skaggs, Robert L 

Review of anhydrous zirconium-hafnium separation techniques. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 8963) 

Bibliography: p. 22-25. 

Supt. of Docs, no.: I 28.27:8963. 

1. Separation (Technology). 2. Zirconium tetrachloride. 3. Hafni- 
um tetrachloride. I. Rogers, Daniel T. II, Hunter, Don B. III. United 
States. Bureau of Mines. IV. Title. V. Series: Information circular 
(United States. Bureau of Mines) ; 8963. 



■--Tf^^a5LjU4 [QD63.54] 622s [669'.735] 83-20940 



CONTENTS 



Page 



1 


2 


4 


5 


7 


10 


12 


13 


16 


17 


17 


18 


18 


21 


22 



Abstract 

Introduction 

Separation techniques based on relative volatility 

Thin-film sublimation 

Extractive distillation from molten salts 

High-pressure liquid-vapor distillation 

Chemical methods of separating hafnium from zirconium 

Preferential reduction of ZrCl^ 

Fluoride redox exchange 

Chloride-oxide exchange reaction 

Preferential decomposition of alkali metal salts 

Differential oxidation of chlorides 

Ranking of processes 

Conclusions 

References 

ILLUSTRATIONS 

1. Diagram of fractional sublimation apparatus used by Goldberger and Gillot.. 6 

2. Diagram of Spink extractive distillation process 8 

3. Relationship between decomposition temperature of hexachlorozirconates and 

metal ion radius for selected alkali and alkaline earth elements 9 

4. Pechiney process for extractive distillation 10 

5. Vapor pressure of ZrCl4 and HfCl4 11 

6. Density of coexisting liquid and vapor phases of ZrCl4 and HfCl4 11 

7. Apparatus used by Ishizuka to separate HfCl4 from ZrCl4 12 

8. Pressure of the tetrachloride gas over HfCl4(s), ZrCl4(s), and lower chlo- 

rides over Zr 15 

9. Thermal stability of the lower chlorides of zirconium 15 

TABLES 

1. Typical analysis of crude ZrCl4 3 

2. ASTM specification B349-73 for nuclear-grade zirconium sponge 4 

3. Chemical requirements for reactor-grade hafnium metal 4 

4. Physical constants of hafnium and zirconium tetrachlorides 4 

5. Comparison of the thermal stability of the alkali chlorozirconate and chlo- 

rohaf nate compounds 10 

6. Operating conditions and results for a three-stage separation of HfCl4 

from ZrCl4 by the method of Frampton and Feldman 16 

7. Comparison of the distillation processes for separating H.fCl4 from ZrCl4... 19 

8. Comparison of the chemical processes for separating HfCl4 and ZrCl4 20 



t/F JAH 



nwfi 





UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 




A 


angstrom 


kj 


kilo joule 


atm 


atmosphere 


kJ/mol 


kilo joule per gram mole 


°C 


degree Celsius 


kPa 


kllopascal 


cm 


centimeter 


lb 


pound 


cm^/mol 


cubic centimeter per 
gram mole 


Ib/h 


pound per hour 






m 


meter 


cm/h 


centimeter per hour 










mm 


millimeter 


ft 


foot 










ym 


micrometer 


g 


gram 










mm Hg 


millimeter of mercury 


g/cm^ 'h 


gram per cubic cen- 








timeter per hour 


mln 


minute 


h 


hour 


mol pet 


mole percent 


in 


Inch 


pet 


percent , usually weight 
percent 


K 


Kelvin 










ppm 


part per million 


kcal/mol 


kllocalorle per gram 








mole 


psl 


pound per square Inch 


kg/cm^ 


kilogram per square 








centimeter 







REVIEW OF ANHYDROUS ZIRCONIUM-HAFNIUM SEPARATION TECHNIQUES 

By Robert L, Skaggs, Daniel T. Rogers, and Don B. Hunter 



ABSTRACT 

Sixteen nonaqueous techniques conceived to replace the current aque- 
ous scheme for separating hafnium and zirconium tetrachlorides were re- 
viewed and evaluated by the Bureau of Mines. The methods are divided 
into two classes: separation by fractional volatilization of the tet- 
rachlorides, which takes advantage of the higher volatility of hafnium 
tetrachloride; and separation by chemical techniques, based on differ- 
ences in chemical behavior of the two tetrachlorides. 

The criteria used to evaluate separation methods were temperature, 
pressure, separation factor per equilibrium stage, complexity, compat- 
ibility with existing technology, and potential for continuous oper- 
ation. Three processes were selected as being most promising: 
(1) high-pressure distillation, (2) extractive distillation from a 
molten salt, and (3) preferential reduction of gaseous ZrCl4 . Any of 
the proposed nonaqueous Hf-Zr separation schemes must be supplemented 
with additional purification to remove trace impurities. 



'Metallurgist, Boulder City Engineering Laboratory, Bureau of Mines, Boulder City, 
NV (now with Engineering Department, University of Nevada, Las Vegas, NV) . 

^chemist, Boulder City Engineering Laboratory, Bureau of Mines, Boulder City, NV 
(now with Albany Research Center, Bureau of Mines, Albany, OR) . 

■^Metallurgist, Boulder City Engineering Laboratory, Bureau of Mines, Boulder City, 
NV (now with Albany Research Center, Bureau of Mines, Albany, OR). 



INTRODUCTION 



This Bureau of Mines report focuses on 
one aspect of the problem of producing 
nuclear-grade zirconium, via zirconium 
tetrachloride, in a nonaqueous system. 
In 1981, reports of promising, nonaqueous 
Hf-Zr separation work in France and Japan 
led the Bureau to initiate a project to 
reexamine the field. During 1982 a lit- 
erature search covering 1955-82 was con- 
ducted. The search included HfCl4-ZrCl4 
separation techniques and related thermo- 
dynamic data and phase equilibria. Six- 
teen separation processes were critically 
evaluated and rated by a semiquantitative 
method based on cost parameters. On the 
basis of this evaluation, three processes 
were identified as promising candidates 
for commercial-scale separation. The 
three processes are — 

1. High-pressure liquid-vapor 
distillation. 

2. Extractive distillation at 1 atm. 



deposits, with Zr:Hf ratios averaging 
about 50 to 1 (40) . The most common 
sources of zirconium are zircon (Zr02 
•Si02) and baddeleyite (Zr02). Zircon, 
the more abundant of the commercially 
important minerals, occurs domestically 
in beach sands of Florida but is also im- 
ported from Australia and South Africa 
(40). 

The cost of nuclear-grade zirconium in- 
got, $11.60/lb in January 1983, is con- 
sidered high for such a plentiful ele- 
ment, but there are several reasons for 
the high cost: 

1. The zircon ore is very stable. Al- 
though a number of techniques have been 
proposed for unlocking the ore, only 
three have achieved widespread use: 

a. Conversion to ZrCN in an elec- 
tric arc furnace followed by 
chlorination. 



3. Separation by the preferential re- 
duction of ZrCl4(g) to ZrCl3(s). 

The techniques reviewed were limited to 
Hf-Zr separation. In no case was the 
problem of the removal of trace elements 
considered. The evaluation is incomplete 
in that it does not address this impor- 
tant question. Any nonaqueous Hf-Zr 
separation scheme will entail the use of 
additional purification to achieve trace 
element removal. The most promising 
technique for accomplishing this is 
molten-salt scrubbing. 

Zirconium, once considered a rare ele- 
ment, is actually common in the earth's 
crust. Current estimates indicate that 
zirconium makes up 0.028 pet of the lith- 
osphere and is about as plentiful as car- 
bon and much more abundant than the less 
expensive metals copper, nickel, lead, 
and zinc (37) .^ Zirconium is always 
accompanied by hafnium in its natural 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



b. Direct carbochlorination in a 

gas-solid reactor. 

c. Caustic fusion. 

In the process used by Teledyne Wah 
Chang, OR, and by Western Zirconium, UT, 
carbochlorination is employed twice: 
first to unlock the ore, and a second 
time to convert the oxide back to the 
chloride form after aqueous solvent ex- 
traction separation of hafnium. 

2. The hafnium content of nuclear- 
grade zirconium must be less than 100 
ppm. Hafnium has no perceptible effect 
on strength, ductility, or corrosion re- 
sistance, but it negates the utility of 
zirconium in a nuclear reactor. Nuclear- 
grade zirconium must have a low neutron 
absorption cross section (a^, = 0.18 barn 
in zirconium) , but the presence of even 
small amounts of hafnium (Oq = 105 barns) 
greatly increases absorption of neutrons. 
Separation of these two metals is costly 
because their chemical properties are 
nearly identical. 



3. The presence of other minor impuri- 
ties not only affects the ductility and 
corrosion resistence of zirconium, but 
also increases neutron absorption. Fis- 
sionable elements such as uranium and 
thorium are particularly detrimental be- 
cause of the degradation of mechanical 
properties that can occur on exposure to 
radiation. 

Since mineral acids other than Hf will 
not attack, zircon (6^, 60 ) , traditional 
extraction processes rely on high- 
temperature treatment in the presence of 
carbon and chlorine. In these processes 
zircon is reacted with carbon in an arc 
furnace at 3,500° C to form zirconium 
carbonitride, while silicon is driven off 
as SiO gas. The carbide then reacts exo- 
thermally with CI2 to form zirconium tet- 
rachloride (ZrCl^), which is condensed in 
a separate nickel-lined chamber. 

The present commercial process (carbo- 
chlorination) involves treatment of zir- 
con sand-carbon mixtures with chlorine at 
1,150° C in a f luidized-bed reactor (59, 
61) . The silicon tetrachloride byproduct 
is readily separated from the less vola- 
tile ZrCl^ . Table 1 shows a typical 
analysis of the crude ZrCl4 product. 

TABLE 1. - Typical analysis of crude 
ZrCl^ , ' percent 



steel vessel at 
frit: 



600° to 650° C to form a 



Element 


ZrCl4 


Zr 


metal basis 


Zr 


38.3 
.84 
.09 
.19 
.03 
.08 
.01 
.008 
.03 
.5 
Balance 




Balance 


Hf 

Al 


2.2 
.23 


Fe 


.5 


P 


.08 


Si 

Th 


.2 
.03 


Ti 


.02 


U 

Insoluble in H2O. 
CI" 


.08 
NAp 

NAp 





NAp Not applicable. 

^Supplied by Teledyne Wah Chang, Al- 
bany, OR, Dec. 1981. 

In another commercial method used to 
obtain aqueous process solutions, the 
zircon ore is reacted with dry NaOH in a 



Zr02-Si02 + 4 NaOH -»- Na2Zr03 

+ Na2Si03 + 2 H2O. (1) 

The solid frit is washed with water to 
remove soluble silicates. Nitric or sul- 
furic acid is added to dissolve the zir- 
conium values. Caustic fusion has been 
used for unlocking the zircon ore by Co- 
lumbia National in Pensacola, FL, in con- 
duction with the tributyl phosphate(TBP)- 
nitric acid separation process. TBP in 
n-hexane is used as the organic solvent 
for removing zirconium from the nitric 
acid solution. 

Another aqueous method used commercial- 
ly for achieving Hf-Zr separation is the 
methyl isobutyl ketone (MIBK)-thiocyanate 
process. Thiocyanate solutions in MIBK 
are used for removing hafnium from the 
aqueous phase. The less commonly used 
TBP-nitric acid process produces reactor- 
grade hafnium oxide as a byproduct only 
with great difficulty and is not being 
used commercially at present. Hafnium 
production is important because its high 
neutron absorption cross section makes it 
valuable as a control rod in nuclear re- 
actors. Hence the MIBK-thiocyanate pro- 
cess is preferred. 

After removal of the hafnium, hydrous 
zirconium oxide must be precipitated from 
solution, washed with water to remove im- 
purities, and calcined to the oxide, and 
the oxide must be rechlorinated to the 
tetrachloride. Thus, removal of hafnium 
from zirconium by solvent extraction is a 
costly, energy-inefficient process. 

Reduction of the tetrachloride to pro- 
duce zirconium (or hafnium) metal must 
be carried out in the absence of air, be- 
cause the absorption of oxygen or nitro- 
gen renders these metals brittle. Before 
the invention of the Kroll process, the 
only practical way of preparing ductile 
zirconium metal was to decompose Zrl^ on 
a hot tungsten wire. This is a slow, ex- 
pensive process. The Kroll process was 



developed in the mid-1940' s, and commer- 
cial production of zirconium metal was 
achieved in 1953. 

The Kroll process is a batch technique 
and consists of the following steps: 

1. Sublimation of impure ZrCl4 in an 
inert atmosphere such as argon, to leave 
behind undesirable oxides and other non- 
volatile impurities. 

2. Reduction of the ZrCl4 vapor with 
excess magnesium metal at 825° to 875° C 
to produce zirconium sponge. 



3. Vacuum treatment at 
to remove Mg and MgCl2 



920° to 960° C 
This step is 
carried out with the container inverted 
to allow partial drainage of liquid Mg 
and MgCl2 from the solid 



sponge, 
wall. 



which clings to the 



zirconium 
container 



for structural components within such 
reactors. 

TABLE 2. - ASTM specification B349-73 for 
nuclear-grade zirconium sponge (2) 



Element 



Max 

cone, 

ppm 



Al 75 

B 0.5 

C 50 

Cd 0.5 

CI 1,300 

Co 20 

Cr 200 

Cu 30 

Fe 1,500 

Hf 100 



Element 



Max 

cone, 

ppm 



Mn 50 

Mo 50 

N 50 

Ni 70 

1,400 

Si 120 

Ti 50 

W 50 

U (total). 3.5 



TABLE 3. - Chemical requirements for 
reaction-grade hafnium metal (25) 



Zirconium sponge produced in this way 
meets impurity limits of ASTM specifica- 
tion B 349-73 (2^), shown in table 2. 
Standards for hafnium are not as strict, 
although 2 to 5 pet Zr content is the ac- 
cepted upper limit for hafnium metal. 
The chemical requirements for reactor- 
grade hafnium metal are given by Goodwin 
(25) and are listed in table 3. 

After the zirconium sponge is compacted 
and melted into ingots by the consum- 
able electrode method, it is fabricated 
and used as cladding for the fissonable 
uranium in nuclear reactors (39). Zirco- 
nium and zirconium alloys are also used 



Element 


ppm^ 


Element 


ppm^ 


Al 


50 

<5 

50 

<1 

<10 

<10 

<50 

100 

2<30 

<10 

<10 


Mo 


<10 


B 


N 


20 


c 


<10 


Cd 





500 


Co 


Pb 


<10 


Cr 


Si 


<10 


Cu 

Fe 


Sn 

Ti 


<10 
<10 


H 


W 


50 


Mg 

Mn 


Zr pet. . 


2.25 



'Except zirconium. 
^Vacuum melting would result in 5 to 10 
ppm H. 



SEPARATION TECHNIQUES BASED ON RELATIVE VOLATILITY 

of their 



The close similarity between haf- 
nium and zirconium is reflected in the 



physical constants 
shown in table 4. 



chlorides , 



TABLE 4. - Physical constants of hafnium and zirconium tetrachlorides 



Properties 


HfCl4 


ZrCl4 


Properties 


HfCl4 


ZrCl4 


Sublimation temp'..°C.. 
Triple point: 

Temperature °C. . 

Pressure kPa. . 


317 

432.0 
4,501.3 


331 

437.0 
2,235.92 


Critical point: 

Temperature °C. . 

Pressure kPa. . 

Volume cm-' /mol . . 


449.2 

5,776.12 

303.6 


505.0 

5,766.31 

319.3 



'Lange (37). 
Source: Denisova (14) for all except sublimation temperature. 



Three separation concepts iJ^S^, 2^, 29- 
32 , 58-59) based on the relative volatil- 
ity of HfCl4 and ZrCl4 have been studied: 

1. Thin-film sublimation at atmospher- 
ic pressure. 

2. Molten-salt distillation at 1 atm 
(101.3 kPa). 

3. High-pressure fractional distilla- 
tion at 40 to 60 atm (4,050 to 6,080 
kPa). 

THIN-FILM SUBLIMATION 

HfCl4 and ZrCl4 have a relative vola- 
tility PHfCu/PzrCU Of 1.9 at 250° C, 
enabling separation at 1 atm. However, 
sublimation columns do not operate as 
efficiently as countercurrent fractiona- 
tion columns, because the solids adhere 
to the surfaces and thus do not flow 
countercurrent to the gas. To overcome 
this problem, mechanical transfer of sol- 
ids down the column must be accomplished. 

The vapor pressures of solid ZrCl4 and 
of HfCl4 may be represented by equations 
2 and 3 ( JJ- ) : 

log Pzrci4 = 11.4632 - ^^^ 
(below 426° C) , 



where 



a = 



(2) 



log PHfci4 = 11.6726 - ^-^ (3) 



(below 412° C), 

where P is in mm Hg and T is in kelvins. 

As the relative volatility of these two 
tetrachlorides is nearly constant over 
the temperature range 150° to 350° C, the 
Fenske -Underwood equation (51) can be 
used to estimate the number of theoreti- 
cal plates needed to achieve the desired 
separation, under a condition of total 
reflux : 



n = 



Xi/Xo = 



1 . (Xi/X2)p 



(4) 



relative volatility (ap- 
proximately 2 in the 
range of interest, 150° 
to 350° C), 

number of theoretical 
plates required, 

ratio of mole fraction of 
more volatile component 

(1) to mole fraction of 
less volatile component 

(2) on a lower plate o 
and an upper plate p, 
respectively. 



This approach predicts approximately 30 
theoretical plates for the separation. 

The concept was explored in 1949 by 
Plucknett, Hansen, and Duke (54) , who at- 
tempted to separate zirconium and hafnium 
tetrachlorides in a 10-plate column with 
mechanical transfer of the solid material 
down the column. Practical separation 
was not accomplished because only the 
surface of the solids came into equilib- 
rium with the traveling gases. 

The opposite situation, stationary sol- 
ids and an inert gas as a carrier for the 
volatilized tetrachlorides, was investi- 
gated by Jacque and IXimez (32) in 1967. 
Temperature of the heated column was 300° 
to 400° C at the bottom, decreasing to 
150° to 230° C at the top; column height 
was 2 m, and column ID was 16 mm. The 
authors stated that ZrCl4 with an HfCl4 
content of 250 to 1,000 ppm could be pro- 
duced by repeating the batch process sev- 
eral times. The authors postulated a 
process capable of producing 0.3 kg of 
"dehafniated" ZrCl4 in a 4-h cycle. A 
bundle of parallel columns, 8 m long and 
10 mm in ID, with a total cross section 
of approximately 80 cm^ , was required. 

A process was devised by Goldberger and 
Gillot (22^, 24 ) to overcome the disadvan- 
tages of the two solid-gas fractiona- 
tion concepts described above. A column 
4.5 cm in ID by 1 m long, maintained at 



1 



280° C, was filled with inert glass 
spheres of 0.42- to 0.59-iDm diam, which 
moved down the column by gravity at a 
controlled rate of 50 cm/h. A drawing of 
the apparatus is shown in figure 1 . The 
crude ZrCl4 was vaporized in a chamber at 
the bottom of the column and carried up- 
ward by a stream of nitrogen gas. The 
HfCl4-rich vapor was condensed at the top 
of the column. Fresh inert material was 
fed into the top of the column as re- 
placement for that withdrawn at the bot- 
tom; the purified ZrCl4 was vaporized 
from the glass spheres and condensed in a 
separate chamber. The patent by Goldber- 
ger and Gillot also covered separation by 
means of a temperature gradient without 
the nitrogen gas stream. 

In this patent (24) , it is stated that 
a mixture of 2.64 g ZrCl4 and 2.51 g 
HfCl4 (59.3 mol pet ZrCl4), after frac- 
tionation for 3 h in the column de- 
scribed, had the following compositions: 



The repeated sublimation and condensa- 
tion of the tetrachlorides concentrates 
the more volatile HfCl4 in the upper part 
of the column. Goldberger and Gillot 
found a height equivalent to a theoreti- 
cal plate of 22 cm and postulated that 
the column length would have to be 4.2 m 
to achieve the desired purification of 
the ZrCl4 , but no practical work was done 
to confirm this. The process has the 
following advantages and disadvantages: 

1. Process operates at atmospheric 
pressure; the need for high-pressure pro- 
cess components such as valves and seals 
is eliminated. 

2. Temperatures do not exceed 350° C. 
Thus , material requirements are less 
stringent. 

3. Process is simple in operation and 
could be operated continuously if 
required. 



Height of sampling point Composition of 



above bottom of heating 


sample, mol 


jacket, cm 
12 


ZrCl4 
68.7 + 0.8 


32 


57.3 + 3.0 


52 


38.5 ± 4.0 



Condensed 
oroduci. 




Condensed 
^ product. 
ZrCI,-rich 



Panicle conveye 



FIGURE !• - Diagram of fractional sublimation ap- 
paratus used by Goldberger and Gillot (24). 



4. No process reagents are used, and 
no byproduct streams are created. 

5. The main disadvantage is tempera- 
ture control, particularly if larger 
diameter columns are used, because of the 
difficulty in achieving uniform radial 
temperature. 

6. Secondary operations are required 
for stripping the inert solids exiting 
the bottom of the column, so that carrier 
material can be recycled. 

7. The diffusional nature of the pro- 
cess requires that the uniform film 
thickness be less than 1 ym in order to 
achieve equilibrium between the solid 
and vapor within a realistic time peri- 
od. This implies a very inefficient uti- 
lization of column volume. The mass 
throughput per equilibrium stage for the 
work described was approximately 0.004 
g/cm^*h, compared to a corresponding val- 
ue of 5,800 times greater for the liquid- 
vapor separation of Ishizuka (29) . 

8. The need to protect the hydroscopic 
solid from contact with the atmosphere 



complicates the problem of column 
operation. 

EXTRACTIVE DISTILLATION 
FROM MOLTEN SALTS 

Researchers (15-16 , 24 , 43 ) have 
pointed out the technical difficulties 
associated with the high-pressure liquid- 
vapor distillation of ZrCl4-HfCl4 mix- 
tures. The operating pressures needed to 
achieve liquid-vapor distillation are in 
the range of 40 to 60 atm and require the 
use of special high-pressure components. 
A serious drawback associated with high 
operating pressure is the difficulty in 
achieving continuous operation that in- 
volves feeding a solid into the system. 
Only a narrow operational temperature 
range exists between the triple point 
(437° C) and the critical point (505° C) 
of ZrCl4. This condition places a strin- 
gent temperature control requirement on 
the separation system. 

A number of attempts have been made to 
achieve distillation using solutions of 
(Zr,Hf)Cl4 in molten salts. The use of 
molten salts decreases the activity of 
the tetrachlorides and permits separation 
at atmospheric pressure and at tempera- 
tures below 400° C. 

In 1976, Spink (_58) obtained a patent 
covering the distillation of a feed mix- 
ture of 63 mol pet tetrachlorides, 29 mol 
pet KCl, and 8 mol pet NaCl. This is the 
ternary eutectic composition with an in- 
variant freezing temperature of 218° C. 
As shown in figure 2, the crude feed eu- 
tectic solution is fed into a distilla- 
tion column operating between 325° C at 
the top and 400° C at the bottom. Fifty 
theoretical plates are specified in or- 
der to obtain the desired end products, 
nuclear-grade ZrCl4 and commercially pure 
HfCl4. This number of plates corresponds 
to a relative volatility of 1.7, which is 
assumed constant over the entire range 
of operating temperature. Reflux of the 
HfCl4-rich overhead vapor stream and re- 
cycle of the stripped ZrCl4-rich salt 
bottoms are included in the scheme. This 
necessitates the handling of hot molten- 
salt streams ; the feed and reflux flows 



can be transferred by gravity, but the 
recycle stream must be pumped. Pumping 
of molten salts in the range of 300° to 
400° C requires special consideration. 
Alternatively, the molten salts could be 
transferred using inert gas pressure. A 
variation that avoids molten salt is to 
add the solvent salt at the top of the 
column as a granular solid. To eliminate 
the need to crush the solidified salt, 
Spink has experimented with "prilling" or 
dropwise solidification. 

Removal of the product tetrachloride 
from the overhead and bottom streams is a 
major problem in molten-salt distilla- 
tion. Spink and Jonasson (59) concluded 
that a combination of high temperature 
and vacuum was necessary to completely 
remove ZrCl4 from the bottoms salt be- 
cause of the high stabilities of K2ZrClg 
and Na2ZrCl6. 

Three possible approaches for removal 
of the ZrCl4 from the bottom stream 
will be considered in the following 
paragraphs: 

1. Molten-salt electrolysis . — Electro- 
winning of zirconium from the ZrCl4-rich 
bottoms stream is an attractive approach 
in that the solvent salt mixture would 
be returned to the mixing tank with its 
original composition. Spink and Vijayan 
(60) , however, have demonstrated that the 
tetrachloride-rich solution is unsuitable 
for electrowinning because of the high 
vapor pressure exerted. For this reason, 
only dilute ZrCl4 solutions are used in 
electrowinning. Martinez and Couch (41) 
were able to produce ductile zirconium 
crystals from NaCl melts, but the level 
of zirconium was only 2 pet. It is ap- 
parent that the bottom stream of the 
distillation column cannot be treated 
directly on a commercial scale by molten- 
salt electrolysis. Dilution is a possi- 
bility, but an extremely large electro- 
lytic recovery vessel would be needed. 

2. Partial thermal stripping with re- 
cycle . — Spink and Jonasson (59) have sug- 
gested that the addition of MgCl2 to the 
solution salt might increase the activity 
of ZrCl4 according to the reaction 



K2ZrCl6 + MgCl2 ->■ K2MgCl4 
+ ZrCl4 ( g ) , 



(5) 



as reported by Tverskov and Morozov ( 63 ) . 
Examination of the KCl-NaCl-MgCl2 ternary 
system (32) shows three double salts: 
KCl'MgCl2~mp - 490° C) , NaCl«MgCl2 (dec 
^ 465° C) and 2NaCl-MgCl2 (dec ^ 485° C) 
(36) (mp = melting point; dec = decompo- 
sition point.) A ternary eutectic point 
occurs at 385° C and approximately 46 mol 
pet MgCl2, 21 mol pet KCl, and 33 mol pet 
NaCl. ZrCl4 and MgCl2 form a simple eu- 
tectic system with the invariant point 
at 426° C and 94 mol pet ZrCl4 . Thus, 
the addition of MgCl2 to the metal chlo- 
ride solvent not only increases the 
activity of ZrCl4 by incorporating KCl 



and NaCl into double salts , but also low- 
ers the melting point of the 
depleted return solvent salt. 



ZrCl4- 



Dutrizac and Flengas (15-16, 18 ) made a 
systematic study of the stabilities of 
double salts of zirconium and hafnium 
with alkali and alkaline-earth chlorides 
and found a relationship between decompo- 
sition temperature and the radius of the 
metal ion. Decomposition temperature was 
defined as the temperature at which the 
vapor pressure of the ZrCl4 over the dou- 
ble salt was equal to 1 atm. Figure 3 
shows a plot of decomposition temperature 
versus metal ion radius. The smaller 
metal ions are more able to pull a cova- 
lently bonded Cl~ ion from the octahedral 
ZrClg^- ion. The addition of Li'^, Mg2+, 



HfCl4-ricti gas 



Crude (Zr, Hf]CI 



330°- 
340°C 



Feed tank 



1 



Feed stream 



Overhead vapor 




32 5°C 




Makeup'salt 



Retlux melt tank 



Retlux line 

Fractionation column- 
50 theoretical plates 
up to 90 actual plates 



400°C 



J 



ZrCl4 vapor, 
50 ppm HfCU 

^ 



Bottoms, ZrCl4-rich salt 



4 2 0^ 4 5 °C 



Vaporizer 



Recycle line 



I Purge salt 
FIGURE 2. - Diagram of Spink extractive distillation process (58). 





l,OUU 




1 


1 


\ . 


►Cs+ 


^ 










Rb+ X 




< 

a. 

lU 

a. 
5 


1.100 


— 




Na+ . 


>&/ 




- 


900 







A 


> 




z 
C 

M 
O 

a. 
2 
O 

o 

UJ 

O 


700 
500 


— 


-V 




r. 2 + 

^ Ba 
1 





0.5 



1.0 



1.5 



2.0 



METAL ION RADIUS, A 

FIGURE 3, • Relationship between decomposition 
temperature of he xachloro zircon at es and metal ion radi- 
us for selected alkali and aJkalineearth elements. (Af- 
ter Morozov and Sun (45)) 

or Ca^"^ to the melt breaks up the ZrCl5 2- 
ion and increases the activity of ZrCl4. 
Such a solvent melt, high in LiCl and 
MgCl2, should be easily stripped at mod- 
erate temperature. 

3. Reduction in the salt solution with 
electrolysis of the product salt . — The 
entire bottoms stream may be fed directly 
into a vessel, where zirconium is recov- 
ered from the molten salt by reducing the 
ZrCl4 to the metal, which precipitates. 
The zirconium-depleted salt is drained 
from the reduction vessel. During the 
reduction step, the original eutectic 
salt composition is altered due to the 
generation of either MgCl2 or NaCl, de- 
pending on the reducing metal agent used. 
Electrolysis of the resulting NaCl-KCl- 
MgCl2 salt solution could be used to re- 
store the composition and at the same 
time generate reductant metal. Based on 
the relative oxidation potentials, the 
resulting metal would be predominately 
magnesium. 

The above practice would provide a 
completely closed system. The distil- 
lation column becomes an integral com- 
ponent in a closed system consisting 
of extractive distillation column, 



reduction reactor, and a molten-salt 
electrolysis cell for the recovery of 
the high-magnesium alloy. Opportunities 
exist for energy conservation by the 
use of hot metal and hot salt transfer 
from stage to stage. However, it is not 
consistent with existing practice, which 
involves reduction of the pure tetra- 
chloride with magnesium. New techniques 
and equipment would be required if re- 
duction in molten-salt solution were 
adopted. 

The process developed by Besson (_5) for 
Pechiney Ugine Kuhlmann, whose subsidiary 
Cezus is reported by Brun (^) to be using 
it commercially, is based on mixing ZrCl4 
and HfCl4 with molten aluminum chloride 
and potassium chloride and distilling the 
mixture at atmospheric pressure. It is 
referred to as the Cezus-Pechiney pro- 
cess. The resulting ZrCl4 contains less 
than 50 ppm HfCl4. This process has been 
termed "extractive distillation" by its 
developers and is similar to the process 
investigated by Spink. Two critical dif- 
ferences are evident: 

1. The ratio of AICI3 and FeCl3 to KCl 
in the melt must be maintained above 
0.94:1, and preferably between l.OA to 
1.10:1, by periodic addition of AICI3 or 
FeCl,. 



2. After the HfCl4 has been removed 
4 , the latter is stripped 



from the ZrCl 
out of the solvent salt by a nitrogen 
stream and condensed. The molten solvent 
salts are recirculated to the top of the 
column so that the operation is continu- 
ous. Figure 4 shows a schematic repre- 
sentation of the process. 

Although the patent claim of Besson (_5) 
cites use of either FeCl3 or AICI3 in the 
solvent salt, in the process described by 
Brun (8^) only AICI3 is used. 

Based on the correlation of Dutrizac 
and Flengas (15), FeClj or AICI3 should 
be quite effective in unlocking the 
ZrClg2- con^lex. They have shown that 
the stability of the metal chlorozircon- 
ate will vary as (r^ + rci)2/q^, where r^, 
is the ionic radius of the metal, Xq\ is 



10 



Recycle salt 



Condenser 



Crude feed 




Reboiler 



Reservoir 



Salt pump 



FIGURE 4. - Pechiney process for extractive 
distillation (5). 

the covalent radius of chlorine in the 
ZrCl5 2~ complex, and q^ is the charge on 
the metal ion. Table 5 shows the decom- 
position temperatures of several alkali 
chlorozirconate and chlorohafnate com- 
pounds. Because of the relatively small 

TABLE 5. - Comparison of the thermal 
stability of the alkali chlorozir- 
conate and chlorohafnate compounds 



Compound 


Dec 


omposition 


Reference 




temp 


eraturej °C 




Li2ZrCl6.. .. 




501 


3 


Li2HfCl6 




513 


3 


Na2ZrCl6 




634 


3 


Na2HfCl6 




-648 


35 


K2ZrCl6 




831 


3 


K2HfCl6 




863 


34 


Rb2ZrCl6.... 




904 


3 


Cs2ZrCl6. ... 




1,040 


3 


Cs2HfCl5 




953 


3 



P(Zr,Hf)CI = 1 atm. 



size and trivalency of Fe^"*" or Al^"*", the 
removal of ZrCl4 should be accomplished 
readily. Besson (5) reports that at 
500° C and 13 mm Hg (1.7 kPa) , the resid- 
ual ZrCl4 was reduced to 0.6 g/100 g 
KAICI4. No mention was made of FeCl3 or 
AICI3 contamination of the product tetra- 
chloride. This should occur because of 
the volatilities of these two substances, 
but the product reported by Brun (8) 
yields a nuclear-grade sponge. 

This extractive distillation process, 
termed by its inventors the "S" process, 
is being used in a pilot plant, replacing 
the MIBK-thiocyanate process. However, 
nothing in either the patent claim by 
Besson or the article by Brun indicates 
how contamination of the purified ZrCl^ 
by AICI3 is prevented. There must be a 
practical reason for using AICI3 instead 
of FeCl3, because ASTM specifications 
permit 1,500 ppm Fe but only 75 ppm AICI3 
in the zirconium metal. 

The role of trace impurity chlorides 
must be considered in any molten-salt ex- 
traction process. The main impurities 
are the chlorides of Fe, Al, Si, P, and 
Ti (table 1); in the Cezus process (7^), 
these are removed by a preliminary subli- 
mation of the crude ZrCl4-HfCl4. An al- 
ternative method is molten-salt scrubbing 
of the ZrCl4 , described by Spink (53) . 
Greenberg (26) and Frey (20) describe 
other patented methods for selective im- 
purity removal, Greenberg claims that 
aluminum halides can be removed by dis- 
tilling the ZrCl4 through CaCl2. Frey 
states that the use of highly viscous oil 
that carbonizes below the sublimation 
points of ZrCl4 and HfCl4 will remove 
FeCl3, A patent was issued to Ross (55) 
for removal of CO, COCI2, and CI 2 from 
crude ZrCl4, The impure ZrCl4 was dis- 
solved in a KCl-NaCl bath partitioned 
into chambers, and the purified tetra- 
chloride was removed as a vapor. 

HIGH-PRESSURE LIQUID-VAPOR DISTILLATION 

Distillation techniques require heavy- 
duty components necessary to withstand 
pressures of 587,6 to 881.4 psi (4,050 to 



11 



6,080 kPa) and temperatures up to 505° C. 
Materials of construction must be resist- 
ant to ZrCl4 , Hf CI4 , and impurity chlo- 
rides. Despite these requirements, sev- 
eral processes have been devised for the 
high-pressure liquid-vapor separation of 
ZrCl4 and Hf CI4 . 

In 1958, Bromberg (7^) patented a method 
for purification of ZrCl4 by fractional 
distillation. The patent claims that the 
temperature should be between 455° and 
520** C at the bottom of the column and a 
minimum of 440° C at the top. The criti- 
cal temperature for ZrCl4 was probably 
not known in 1958 when the Bromberg pa- 
tent was written because 520° C is above 
the critical temperature for ZrCl4 (505° 
C). A line leading from the top of the 
column enables the more volatile HfCl4 to 
be condensed in a receiving vessel. The 
purified condensed ZrCl4 is collected in 
a receiving vessel at the bottom. The 
impure ZrCl4 (1.6 pet HfCl4) is vaporized 
from a boiler at the side of the frac- 
tionation column. Valves on all three 
storage vessels enable the HfCl4-rich 
distillate (92 pet HfCl4) and the puri- 
fied ZrCl4 (60 ppm HfCl4) to be withdrawn 
at intervals, and fresh, impure ZrCl4 to 
be added periodically. Preferred operat- 
ing temperatures were 495° C at the bot- 
tom and 460° C at the top. The column 
was constructed of type 316 stainless 
steel, and was 26 ft high by 3 in. in ID. 
Either a plate or packed-column design 
is claimed to be effective. Bromberg 
claimed that a 36-ft column, operating at 
a reflux ratio of 100:1, would produce 
nuclear-grade ZrCl4 in the bottom receiv- 
er, while purified HfCl4 would be taken 
off from the top. 

The narrow operating range of this pro- 
cess was made clear in 1967 when Deni- 
sova, Safronov, Pustil'nik, and Bystrova 
published their study of liquid-vapor 
phase equilibria of ZrCl4 and HfCl4 (14). 
In figure 5, solid-vapor and liquid-vapor 
behavior are shown. The authors' exten- 
sion of the log P versus lO^/x plot into 
the supercritical region is unexplained. 
Figure 6 shows the densities of coex- 
isting liquid and vapor phases at con- 
stant temperature. The temperature at 



TEMPERATURE. C 
496 468 441 




1.20 125 130 135 140 



10''/T. deg'V 



145 1,50 



FIGURE 5. - Vapor pressure of ZrCl^ and HfCI^. 
(After Denisova (14)) 



510 



500 — 



490 — 



480 



UJ 

^ 470 

h- 
< 

ifJ 460 

Q. 

2 



450 — 



440 — 



430 



420 




KEY — I 

C Critical point 



5 10 15 2.0 

DENSITY, g /cm ^ 

FIGURE 6. - Density of coexisting liquid and vapor 
phases of ZrCI^ and HfCI^. (After Denisova (14)) 



12 



the bottom of the column is limited 
by the critical conditions for ZrCl4 
(5,766.3 kPa and 505.0° C) . The lower 
temperature limit of operation is the 
triple point for Hf CI4 , 432° C. 

The necessity for a narrow range of 
operating conditions was also reported by 
Ishizuka (29). The 1974 patent appli- 
cation stated that the temperature of 
the boiler was 469° C, bottom rectifier 
466° C, top rectifier 461° C, and con- 
denser 454° C, at an operating pressure 
of 40 kg/cm2 (3,900 kPa) . Column height 
and ID were 700 mm and 20 mm, respective- 
ly. Figure 7 shows the type of apparatus 
used by Ishizuka. The crude feed to this 
batch process contained 2 pet HfCl4 and 
produced, after 24 h of operation, a con- 
denser product with 32 pet HfCl4 and a 
ZrCl4 boiler product with 50 ppm HfCl4. 
Before the column was frozen to col- 
lect product fractions, the boiler prod- 
uct contained only 8 ppm Hf CI4 . Although 
specification HfCl4 (<5 pet ZrCl4) was 
not obtained, extrapolation to the 26-ft 
column used by Bromberg (7) indicated 
that the Ishizuka column was more 
efficient. 

Operation of a unit for processing 5 
tons of crude ZrCl4 is reported in a Eu- 
ropean patent application by Ishizuka 
(30) . A mild steel column was good for 
20 to 50 runs before it needed substan- 
tial repairs. A second distillation was 
necessary to convert hafnium-rich over- 
head chloride to nuclear-grade HfCl4. 
Removal of impurity chloride was achieved 
by adding of small amounts of NaCl or KCl 
to form nonvolatile complexes with AICI3 
and FeClj. 



fpij Pressure gages (B) 



r 



4XH=^ 



Sampling valve 



. Reflux condenser 



— yU — . — Reflux reservoir 



Overhead product 




KEY 

Heated portion 

— Insulated portion 
q Ttiermocouple 



Sampling valve 



@- 



Packed column 



Reboiler 



Sampling valve 



I I I 

FIGURE 7, - Apparatusused by Ishizuka to separate 
HfCI^ from ZrCI^ (29). 

The narrow operating range for frac- 
tional distillation shown in these pa- 
tents would require close temperature 
control for successful separation of zir- 
conium and hafnium tetrachlorides. 



CHEMICAL METHODS OF SEPARATING HAFNIUM FROM ZIRCONIUM 



Because of the difficulty of separat- 
ing hafnium from zirconium by sublimation 
or fractional distillation, chemical 
methods have been investigated. Unlike 
the methods based on relative volatility, 
chemical separation techniques cannot be 
easily categorized. As a broad general- 
ity, hafnium compounds are slightly more 



stable than the corresponding zirconium 
analogs . This is probably because the 
Hf-X bond is stronger and displays a more 
covalent character than the Zr-X bond. 
These small differences have been ex- 
ploited in the separation of hafnium from 
zirconium by chemical methods . 



13 



A number of schemes have been proposed. 
Each is unique, so the chemical separa- 
tion methods must be treated individual- 
ly. Several of the most promising candi- 
date processes follow: 

Preferential reduction of ZrCl4 (Newn- 
ham - 1957) (46). 

Fluoride-redox equilibrium (Megy - 
1979) (44). 

Chloride-oxide exchange (Chandler - 
1966) (9^). 

Preferential decomposition of salts 
(Flengas and Dutrizac - 1977) (18). 



The reduction reaction may be repre- 
sented by 



Differential oxidation of 
(Berl - 1961) (4). 



chlorides 



Each process has inherent advantages 
and disadvantages. The high separation 
factor and closed cycle nature of the 
Nevmham process are offset by the neces- 
sity of handling pyrophoric solids. The 
Megy process has the highest separation 
factor, but the use of fluoride and the 
lack, of compatibility with existing Kroll 
or electrolytic technology are serious 
disadvantages . 

PREFERENTIAL REDUCTION OF ZrCl4 

In 1957, Newnham (4_6) obtained a patent 
for the separation of HfCl4 from ZrCl4 
based on the observation that ZrCl4 is 
more easily reduced to the lower chlo- 
ride form than is Hf CI4 . For example, at 
427° C (700 K) the Gibbs energy change 
for the reaction (23) 

HfCl3(s, + ZrCl4(g) > HfCl4(g) 

+ ZrCl3(s) (6) 

is -22 kcal/mol (-92 kj/mol). The lower 
chloride of zirconium remains in the con- 
densed form, while HfCl4 and unreacted 
ZrCl4 may be sublimed. The separation is 
much more effective than one based on the 
relative volatilities of HfCl4 and ZrCl4. 



Zr(s) + 3 ZrCl4(g) ^ 4 ZrCljcs) 



(7) 



A number of reducing agents may be used, 
but zirconium metal is preferred because 
no impurities are introduced into the 
system. The more volatile HfCl4 and the 
unreacted ZrCl4 remain in the gaseous 
form. 

ZrCl3 is subsequently heated to 420° to 
460° C, where it disproportionates: 

2 ZrCl3(s) > ZrCl2(s) + ZrCl4(g). (8) 

The low-hafnium ZrCl4 product is recov- 
ered, and the resulting ZrCl2 solids are 
recycled as a reducing agent in subse- 
quent stages: 

ZrCl2(s) + ZrCl4(g) -^ 2 ZrCl3(3), 

340° to 420° C. (9) 

The patent proposes a process that is 
closed and cyclic. 

In 1959, Newnham obtained a second pa- 
tent (47) that extended the original con- 
cept to carry out the reduction in a 
molten-salt medium, such as AlCl3-NaCl, 
LiCl-KCl, or other mixtures containing at 
least one alkali chloride salt. The 
molten-salt medium keeps the temperature 
close to the optimum required for selec- 
tive reduction. In addition to separa- 
tion of ZrCl3 and HfCl4 ^y volatility, 
the patent claims that this separation 
can be carried out through decantation or 
filtration, as ZrCl3 is a solid in a liq- 
uid medium. 

A related patent was issued in 1973 to 
Larsen and Gil-Arnao (38) . In this case 
the crude ZrCl4 is reacted with a metal- 
lic reducing agent (Al or Zr) in a pure 
molten AICI3 medium. Whereas Newnham in- 
sisted on the presence of an alkali chlo- 
ride salt to maintain atmospheric pres- 
sure, Larsen implies that the resultant 



14 



advantage of a faster reaction at lower 
ten^erature (260° C) is more important 
than avoiding higher pressure, which 
could be as high as 8 atm (810 kPa) for 
pure AICI3 at 260° C (12). Recycling of 
ZrCl2 recovered is the same as in the 
Newnham method. Separation factors dem- 
onstrated by Larsen vary from 5.7 to 
19.8, while Newnham (48) demonstrated 
values up to 200. Frampton and Feldman 
(19) report separation factors from 6.6 
up to 22 for the technique. Separation 
factor (SF) is defined by 



Hf in feed, pet 
Hf in product, pet 



(10) 



Larsen points out that the liquid-phase 
reaction mechanism overcomes the disad- 
vantages of the solid-gas reaction of 
Newnham, where the ZrCl3 coats the sur- 
face of the zirconium metal and impedes 
the 
ZrCl4 
phase 



reaction. With molten AICI3, the 



first forms a blue intermediate 
that is soluble in the melt, so 



that the melt turns blue. Brown ZrCl 



3» 



which is insoluble in molten aluminum 
chloride, forms later, and the molten 
bath becomes colorless, which indicates 
the end of the reaction. Related patents 
are claimed by Newnham (48-49) . 

The disproportionation of ZrCl3 to re- 
generate ZrCl2 is complicated by a series 
of reactions to form nonstoichiometric 
compounds. Shelton and others (10, 54 ) 
summarize the reactions and their temper- 
atures of occurrence, as follows: 

12 ZrCl3(s) > 10 ZrCl2.8(s) 
+ 2 ZrCl4(g), 115° to 300° C. (11) 

10 ZrCl2.8(s) ^ 5 ZrCli.6(s) 
+ 5 ZrCl4(g), 310° to 450° C. (12) 

5 ZrCli.6(3) > 4 ZrCl(s) 
+ ZrCl4(g), 500° to 600° C. (13) 

4 ZrCl(s) -> 3 Zr(s) + ZrCl4(g), 
570° to 700° C. (14) 



The equilibrium ZrCl4 pressures for the 
first two of these reactions are given by 
Copley and Shelton (10) : 

log P = -6,138/T + 13.288, (15) 

log P = -9,870/T + 15.555 (16) 

respectively , where P is given in mm Hg 
and T in kelvins. The latter two reac- 
tions are not important in the currently 
conceived reduction process cycle. 

Shown in figure 8 is a plot of log P 
versus 10^/T for the sublimation of solid 
HfCl4 and ZrCl4 (56). On the same plot 
is shown the decomposition pressure of 



ZrCl 



4(g) 



over lower chlorides of zirconi- 



um. The stability of the lower chlorides 
of zirconium as a function of temperature 
and pressure is shown in figure 9 (56) . 

In 1968, Mauser (42) studied the selec- 
tive reduction reaction occurring in a 
rotating stainless steel reactor filled 
with stainless steel balls. The rolling 
balls crush the particles and break up 
the ZrCl 3 coating that forms on the re- 
ductant and quenches the reaction. From 
this gas-solid reaction study, the fol- 
lowing conclusions were drawn: 

1. The reactor grinding balls were ef- 
fective in eliminating sintering (agglom- 
eration) of the reacting particles. 

2. The dichloride (ZrCl 2) was not an 
effective reductant for ZrCl4. Zirconium 
in the form of sponge, minus 325-mesh 
fines, or machine turnings had to be 
used. Regeneration was carried out at 
900° C in order to drive the dispropor- 
tionation reactions to completion and 
yield finely divided metallic zirconium. 
Improved yields and separations were re- 
ported with recycled zirconium. This was 
attributed to the increased surface re- 
sulting from repeated reduction and dis- 
proportionation. The zirconium became 
increasingly pyrophoric with each cycle. 

3. Zirconium sponge and fines were 
equally effective reductants, but the 
turnings were less effective. 



15 



KEY 

8 ZrCl4(35^z=z=± ZrCl^^g) 

^ 2^30C'90(s}^ Z^29Cl86(s] + ^rCUcg) 

e 2ZrCl2 8 -. ZrClig^g^ + Z^CI^^g) 




3 -1 

10 /T. deg K 



FIGURE 8. - Pressure of the tetrachloride gas over 
HfCL, ,,ZrCi., ,, and lower chlorides over Zr, (After 

4( s)' 4( s )' 

Gjpley and Shelton (10)) 



National Distillers and Chemical Corp. 
obtained the rights to the Newnham pa- 
tents and devoted considerable effort to 
bring the dry process to commercial prac- 
tice. Frampton and Feldman (19) have de- 
scribed this work. Although ZrCl2 is re- 
ported to be a satisfactory reducing 
agent, the temperature at which the pre- 
ferential reduction is carried out is 
critical. In the temperature range 330° 
to 370° C, a nonselective lower chloride 
complex (Zr3Cl8 'Hf CI4 ) is formed and de- 
creases the separation factor. Above 




400 600 

TEMPERATURE. ° C 



1000 



FIGURE 9." Thermal stability of the lower chlorides 
of zirconium, (After Shelton (56)) 

420° C, the disproportionation of ZrCl3 
occurs at an appreciable rate. The ZrCl^ 
formed mixes with Hf CI4 , and the separa- 
tion factor is decreased. These two con- 
ditions restrict the temperature of the 
reduction operation to 400°±20 ° C. The 
close temperature control was obtained by 
immersing the apparatus in a bath of mol- 
ten tin; the authors suggested using 
molten sodium-potassium alloy (NaK) for 
large-scale operations. The equipment 
was constructed of type 316 or 347 stain- 
less steel. To insure thorough mixing of 
the reactants, an anchor-type stirrer 
that scraped the bottom and sides of the 
container and prevented any buildup of 
solids was used. 

Disproportionation of ZrCl3 was car- 
ried out in the range 420° to 460° C and 
yielded product ZrCl4 and regenerated 
ZrCl2. The initial ZrCl2 bed was pre- 
pared by reacting finely divided zirco- 
nium sponge with ZrCl4 vapor at 430° C 
for an extended time and subsequently in- 
creasing the temperature to 460° C to 
cause disproportionation. 

The authors proposed a pilot plant in 
which the reactions would be carried out 
in horizontal tube, screw-fed reactors 
that would produce 25 Ib/h of hafnium- 
free ZrCl^ . The separation factor used 
in the hafnium concentration stage is a 
very conservative 1.6; three to four 



16 



stages of separation were required to 
produce ZrCl4 containing < 100 ppm Hf/(Hf 
+ Zr). 

Frampton and Feldman report a three- 
stage separation using solid feed. Oper- 
ating conditions and results are given in 
table 6. The solid-gas process ( 19 , 46 ) 
studied by Frampton and Feldman has the 
following advantages: 

1. The process is closed and cyclic 
and does not require reagents. The only 
raw materials are crude ZrCl4 and makeup 
zirconium sponge. 

2. The National Distillers work has 
already provided a process scheme with 
material balances and a tentative cost 
estimate. 

3. The process lends itself to contin- 
uous countercurrent operation. 

Disadvantages are — 

1. Extremely close temperature control 
is required. 

2. Reducible impurities, such as 



FeCl 



3» 



(ZrCli.e) 
life. 



will collect in the ZrCl2 
bed and shorten its useful 



3. Although the process is potentially 
continuous, initial designs will probably 

TABLE 6. - Operating conditions and re- 
sults for a three-stage separation of 
HfCl4 from ZrCl4 by the method of 
Frampton and Feldman (19) 



Results 



Time h. . 

Hf content, pet: 

Feed 

Product 

Yield, pet: 

Per stage 

Net 

Separation factor 
per stage 



Temperature range, C 



301-328 



1.1 

2.4 
0.31 

65 
65 

7.7 



302-319 



0.9 

0.29 
0.05 

54 
35 

5.5 



318-338 



3.2 

0.05 
0.01 

77 
27 

4.8 



be batch with considerable manual 
dling of equipment and materials. 



han- 



4, Mechanical agitation is necessary 
to expose fresh zirconium surface to va- 
por. Several techniques are available to 
accomplish this: 

a. Stirred reactor. 

b. Rotating ball mill reactor. 

c. Fluidized-bed reactor. 

5. The effectiveness of ZrCl2 as a re- 
ducing agent is questionable. If Mau- 
ser's observations are correct, a consid- 
erably higher regeneration temperature 
(900° C) will be required in order to 
produce metallic zirconium. 

FLUORIDE REDOX EXCHANGE 

In 1978, Megy (43) improved yields on 
the exchange reaction 



ZrFg^- + Hf J HfF6 2- + Zr 



(17) 



by the addition of molten zinc to dis- 
solve the zirconium metal produced. The 
zinc shifted the reaction to the right 
because zirconium is preferentially dis- 
solved in molten zinc and also increased 
the reaction rate by improving transport 
in the molten zinc so that conversion was 
essentially complete in 5 min. 

The equilibrium constant (Kq) for the 



reaction as a function 
in kelvins) is 



of temperature (T 



log Kq = -1.565 + 4,320/T 



(18) 



for systems using Na2ZrF6 , plus NaCl and 
KCl to lower salt phase melting tempera- 
tures (700° to 900° C). 

A reductant, preferably aluminum metal, 
must be used to convert hafnium and zir- 
conium salts to the metal so that the ex- 
change reaction can occur. The presence 
of aluminum salts does not interfere with 
the separation. 



17 



A similar reaction using chlorides 
rather than fluorides has the disadvan- 
tage of producing intermediate oxida- 
tion states (2+, 3+) for hafnium and 
zirconium. 

Operating in the temperature range with 
molten zinc and fluoride salts poses for- 
midable containment problems. Megy and 
Freund (44) found that during screening 
tests employing temperatures of 800° C 
for 1 h, vitreous quartz, boron nitride, 
alumina, and glassy carbon were attacked. 
Even tungsten and graphite were only mar- 
ginally adequate. Graphite containers 
contributed 100 ppm C to the metal phase 
in a 1-h test (44). For this reason the 
Megy process is of limited interest. 

CHLORIDE-OXIDE EXCHANGE REACTION 

In 1966 Chandler (9) patented a method 
of separating HfCl^ from ZrCl4 by prefer- 
ential conversion of HfCl^ to Hf O2 : 

Zr02(s) + HfCl4(g) > Hf02(s) 

+ ZrCl4(g). (19) 

This is achieved by passing the mixture 
of tetrachloride gases over a bed of Zr02 
and Hf02, where the hafnium preferential- 
ly enters the solid phase. 

Hafnium is removed from the gas phase 
because Hf02 is more stable than Zr02 
relative to the respective chlorides. 
This is a thermodynamic rather than a 
kinetic effect (4^, 2J^) . Equilibrium con- 
stant calculations show little change (Kg 
= 2.3 to 2.8) between 25° and 950° C. Kg 
is determined from 



Ke = 



^ [Hf02] [ZrCl4] 
[Zr02] lHfCl4] 



(20) 



The constant agrees approximately with 
that estimated from Chandler's experi- 
ments (Ke = 5) . 

In the limited work that Chandler per- 
formed, crude ZrCl4 freshly prepared by 
carbochlorination was passed through 
a 15-in bed of crude Zr02 and removed 



two-thirds of the hafnium from the tetra- 
chloride vapor stream (at 950° C over a 
2-h period) . No impurity removal was 
reported. 

PREFERENTIAL DECOMPOSITION 
OF ALKALI METAL SALTS 

Most physical methods for separating 
anhydrous hafnium and zirconium tetra- 
chlorides make use of the higher volatil- 
ity of HfCl4. 

Flengas and Dutrizac (J^, 18 ) have dis- 
covered a separation method in which 
ZrCl4 is the more volatile species. The 
chlorides are converted to alkali metal 
salts, M2ZrCl6 and M2HfCl6. The salts 
are heated, and preferential decomposi- 
tion of the less stable zirconium salt 
occurs at >450° C: 

M2ZrCl6(s) -^ 2MCl(s) + ZrCl4(g). (21) 

Table 5 shows the decomposition temper- 
atures of the double alkali metal chlo- 
rides that were compiled by Flengas and 
and others. Recent studies show that 
potassium is the preferred alkali metal 
cation for the reaction because all oth- 
ers show lower separation factors (34- 
35 ) . The method used (16 , 18 ) involves 
equilibration of 1 mol of HfCl4-ZrCl4 at 
330° C with slightly more than 2 mol of 
KCl held at 450° C. Equilibration takes 
up to 3 days before a separation factor 
of 1.6 is achieved. If the ZrCl4:KCl 
ratio is increased, the separation fac- 
tor decreases. If the reaction time is 
decreased, the tendency of the zirco- 
nium salt to form more quickly than the 
hafnium salt (4^) greatly decreases 
efficiency. 

The disadvantages are — 

1. The 3-day reaction time required 
for static equilibration greatly reduces 
production rates. 

2. Small separation factors (1.6 to 
1.9) for KCl systems require a large num- 
ber of separation stages. 



18 



3. Continuous processes in a packed- 
salt column, where ZrCl4 reacts quickly 
and decomposes quickly at low separation 
efficiency, are plagued by the problem of 
salt swelling; that is, volume change as- 
sociated with the cyclic formation and 
decomposition of the double salt. The 
swelling causes plugging of the column 
(18). 

Although both the reaction kinetics and 
the material throughput might be improved 
by development work, this process is not 
particularly promising. 

DIFFERENTIAL OXIDATION OF CHLORIDES 

Berl (4) has suggested a novel method 
for separating zirconium and hafnium com- 
pounds in a fluidized-bed reactor at tem- 
peratures above 600° C. The basis for 
separation is that the following exother- 
mic reaction for ZrCl4: 

ZrCl4(g) + 02(g) ^ Zr02(s) 

+ Cl2(g) (22) 

proceeds more rapidly than the corre- 
sponding reaction for Hf CI4 . Zr02 Pi^e- 
ferentially builds up in the solid phase, 
while enriching the hafnium content in 
the gas phase. HfCl4 is easily separated 
from product CI 2 by selective condensa- 
tion at 0° to 300° C. 

Funaki and Uchimura (21) confirmed 
the selective reactivity and measured 
the rate constants (k^ , mm Hg/h) as a 
function of absolute temperature. For 
zirconium, 



log kp = 4.25 -5,300/T, 



(23) 



and for hafnium, 

log kr- = 2.9 -4,100/T. (24) 

Calculations show that the rates are 
equal at 615° C. Berl (4_) ran tests at 
temperatures where rate differential 
is small (620° to 800° C) and obtained 
a maximum separation factor of 2.5 at 
620° C. 

Berl (4) used hafnium-free zirconium or 
zirconium oxides as catalysts (seed crys- 
tals). In consideration of Funaki's (21) 
work, the catalyst must be essential to 
the separation at 620° C, where ZrCl4 and 
HfCl4 react at equal rates. 

The method offers no advantages over 
other methods that require numerous 
stages to produce reactor-grade zirconi- 
um. As in other methods, prepurif ication 
to remove AICI3, FeCl3, etc., is neces- 
sary. The most serious problem is the 
requirement that the zirconium catalyst 
be hafnium-free, which makes the approach 
self-defeating. If a countercurrent pur- 
ification process is set up, with ZrCl4 
containing 2 pet HfCl4 fed into one side 
and O2 + Zr02 (2 pct Hf02) into the other 
side, the steps using high-hafnium Zr02 
will be highly inefficient, especially at 
620° C, where rates are nearly equal. 
This result contradicts the results of 
Chandler (9^) , where preferentially oxi- 
dized Hf02 concentrated in the solid 
phase. The separation method is not 
worthy of further study. 



RANKING OF PROCESSES 



All process options must be judged on 
the basis of relative costs. To prepare 
realistic production cost estimates on 
which to base process selection is not 
now possible. It is possible to identify 
critical process characteristics and to 
relate them to costs in a qualitative 
manner. Six characteristics were rated 
for each of the processes studied as 
follows: (--) very unfavorable, (-) un- 
favorable, (0) neutral, (+) favorable, 
(++) very favorable. The results are 



summarized in tables 7 and 8. Caution 
should be exercised in the use of the 
tables because the parameters are not 
equally important. The assignment of 
relative weights would imply an unjusti- 
fied precision for the method. It is not 
intended that the processes be compared 
quantitatively on the basis of point to- 
tals. The tables should be viewed as a 
systematic attempt to consider identifi- 
able factors that contribute to process- 
ing cost. 



19 



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21 



1. Temperature . — Elevated operating 
temperatures involve increased energy 
costs and the use of expensive materials 
of construction. Increased maintenance 
costs from corrosion and deterioration of 
mechanical equipment also occur. 

2. Pressure. — High pressure requires 
the use of heavy-duty components and spe- 
cial fabrication techniques. Added in- 
spection is needed, and an element of 
risk is added. Operation at pressure 
less than 1 atm requires special vacuum 
equipment and fabrication techniques. 

3. Separation Factor . — Separation fac- 
tor is important because it determines 
the number of equilibrium stages required 
to achieve separation. The number of 
equilibrium stages needed reflects on the 
amount of recycle, the number and size of 
reaction vessels, material inventory, en- 
ergy, and operating costs. The cost per 
separation stage may not always be the 
same . 

4. Compatability with Kroll Technol- 
ogy . — The inclusion of a separation pro- 
cess that does not mesh well with the 
Kroll process flow scheme would cause 
the premature loss of usable facilities. 
The design and construction costs for 



replacement equipment are a deterrent for 
such a choice. 

5. Degree of Complexity . — Process com- 
plexity is reflected in the number of 
different steps required and in side 
streams that must be treated. Process 
complexity contributes to costs through 
energy consumption, material inventory, 
labor, and equipment. 

6. Potential for Continuous Process- 
ing. — Although a batch or semibatch pro- 
cess is acceptable, a continuous process 
is more desirable. Improved quality con- 
trol, efficient use of energy, and lower 
labor costs favor the continuous process. 
In some cases the potential for continu- 
ous operation is easy to assess. For 
instance, the continuous operation of a 
high-temperature, high-pressure distilla- 
tion unit would be a difficult undertak- 
ing. The continuous or even intermittent 
introduction and removal of tetrachloride 
from the high-pressure unit is a formida- 
ble task and makes a batch operation a 
more attractive alternative. On the oth- 
er hand, continuous operation is already 
claimed for the extractive distillation 
separation process described by Besson 
(5) and Brun (8). 



CONCLUSIONS 



At least one and possibly three non- 
aqueous Hf-Zr separation processes show 
promise for future commercial operation. 
The economic impact of this development 
on domestic zirconium producers who are 
using the aqueous solvent extraction pro- 
cess is unknown. 

Only two separation processes are now 
being studied. Of these, the extractive 
distillation process of Cezus-Pechiney 
has greater potential for commercial ap- 
plication than the high-pressure dis- 
tillation described by Ishizuka. This 
judgment is based on considerations of 
temperature, pressure, and potential for 
continuous operation. Commercial-scale 
production is already claimed for the 
Cezus process (8). 



Two promising separation techniques are 
not being studied as far as can be deter- 
mined. The Newnham process, based on the 
selective reduction of ZrCl^, was studied 
extensively during the 1960's by both the 
Bureau of Mines and the National Distill- 
ers Corp. The process is simple and has 
high potential for continuous operation. 
The extractive distillation process de- 
scribed by Spink in Canada is similar to 
that used by Cezus, with variations that 
have promise for improved product purity 
and more reliable operation. This work 
was discontinued in 1981 because of lack 
of funding. 

The removal of minor impurities from 
the products has not been solved. The 
Brun description of the Cezus-Pechiney 



22 



process claims a nuclear-grade ZrCl4 
product but does not mention purification 
steps for removal of iron, aluminum, and 
other minor chloride impurities. The 



authors believe that additional purifica- 
tion is necessary regardless of the pri- 
mary Hf-Zr separation process employed. 



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Application of Zirconium. Vancouver, 
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ASTM 6th Internat. Conf. on Nuclear 



14. Denisova, N. D. , E. K. Safronov, 
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23 



1967. London, Institution of Mining and 
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25 



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